Many methods have been devised to strengthen glass articles utilizing the mechanism of a compressive surface skin. The three techniques which have enjoyed the greatest commercial exploitation have been lamination, thermal or chill tempering, and chemical strengthening.
Lamination, involving the sealing together of two glasses having different coefficients of thermal expansion, results in compressive stress in the lower expansion glass and tensile stress in the higher expansion glass when the sealed pair is cooled. That compressive stress increases the mechanical strength of the higher expansion glass if the latter is enclosed within the lower expansion glass. Whereas CORELLE.RTM. tableware, marketed by Corning Glass Works, Corning, N.Y., employs that technique of strengthening, its use has not been widespread because of practical difficulties in tailoring glass compositions which exhibit the proper expansion differentials, which will not adversely react with each other upon contact, and which have compatible viscosity characteristics.
The procedure which has been most widely used commercially for the strengthening of glass articles in thermal or chill tempering. The applicability of that process, however, is restricted to relatively thick-walled articles and the surface compressive stresses developed thereby are relatively low.
Two basic mechanisms have generally been recognized as underlying the chemical strengthening of glass articles.
One form of chemical strengthening comprises the replacement of a smaller alkali metal ion in the surface of a glass article with a larger alkali metal ion from an external source. In commercial practice the glass article containing the smaller alkali metal ions, e.g., Li.sup.+ ions, is immersed into a bath of a molten salt containing Na.sup.+ and/or K.sup.+ ions operating at a temperature below the strain point of the glass. The Li.sup.+ ions from the glass exchange with the Na.sup.+ and/or K.sup.+ ions from the bath on a one-for-one basis. By that exchange the larger Na.sup.+ and/or K.sup.+ ions are crowded or stuffed into the sites formerly occupied by the Li.sup.+ ions, thereby setting up compressive stresses by expanding the surface, and the smaller Li.sup.+ ions pass out into the bath. Because the exchange is carried out at a temperature below the strain point of the glass, relaxation of the glass surface is inhibited. That phenomenon was first clearly elucidated by S. S. Kistler in "Stresses in Glass Produced by Nonuniform Exchange of Monovalent Ions", Journal of American Ceramic Society, 45, No. 2, pp. 59-68, February, 1962. This technique has been limited commercially to special products, most notably ophthalmic lenses, because the low temperature ion exchange requires a relatively long immersion period, e.g., 16-24 hours.
The second basic form of chemically strengthening glass articles, exemplified by U.S. Pat. No. 2,779,136 and termed high temperature exchange, involves the replacement of Na.sup.+ and/or K.sup.+ ions in a glass surface with Li.sup.+ ions from an outside source. Thus, the glass article is immersed into a bath of a molten salt containing Li.sup.+ ions operating at a temperature above the strain point of the glass. The Li.sup.+ ions diffuse into the glass surface and exchange one-for-one with the Na.sup.+ and/or K.sup.+ ions. Because the exchange is carried out at temperatures above the strain point of the glass, relaxation takes place in the glass surface such that a Li.sup.+ ion-containing glass skin is developed. Since this Li.sup.+ ion-containing glass skin exhibits a lower coefficient of thermal expansion than that of the original glass article, when the article is cooled to room temperature the interior glass contracts more than the skin glass, thereby producing a surface compression layer. Where TiO.sub.2 and Al.sub.2 O.sub.3 were present in the glass compositions, the diffusing Li.sup.+ ions reacted therewith to form beta-spodumene crystals, resulting in a surface skin exhibiting a still lower coefficient of thermal expansion, but also being translucent-to-opaque, rather than transparent. Glasses suitable for use in that inventive practice consisted essentially, in weight percent, of
______________________________________ SiO.sub.2 45-80 Al.sub.2 O.sub.3 7.5-25 Li.sub.2 O 0-2 Na.sub.2 O and/or K.sub.2 O 8-15 ZrO.sub.2 0-5 TiO.sub.2 0-15 B.sub.2 O.sub.3 0-2 ______________________________________
The working examples of the patent utilized baths of molten salts containing Li.sup.+ ions operating at temperatures over the interval of 550.degree.-900.degree. C. and for immersion times ranging from two minutes to 15 hours, with the majority of the exemplary baths operating at temperatures between about 600.degree.-825.degree. C. and immersion times of 10-30 minutes. Modulus of rupture values as high as 75,000 psi were reported. This second basic form of chemically strengthening glass articles has not seen commercial service.
In Advances in Glass Technology (1962), pp. 404-411, Plenum Press, New York City, in a section entitled "Strengthening by Ion Exchange", H. M. Garfinkel et al. expanded upon the high temperature chemical strengthening practice of U.S. Pat. No. 2,779,136. They reported treating glass rods consisting essentially, in weight percent of 1.1% Li.sub.2 O, 11.0% Na.sub.2 O, 23.7% Al.sub.2 O, 6.2% TiO.sub.2, 57.2% SiO.sub.2, and 0.5% As.sub.2 O.sub.3 for five minutes in a bath of molten 95% Li.sub.2 SO.sub.4 -5% Na.sub.2 SO.sub.4 (weight percent) operating at 860.degree. C. to yield transparent products exhibiting moduli of rupture values of 70-80 kg/mm.sup.2 (.about.100,000-115,000 psi). X-ray diffraction study of the glass surface indicated the presence of an integral thin layer containing crystals identified as being those of a solid solution of .beta.-eucryptite (classic formula Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2) and quartz. No commercial application of that chemical strengthening technique has been reported, probably because the glass was too viscous to be melted in conventional, continuous-melting tanks such as are used for making drinkware, flat glass, and container glass, and the fact that an immersion temperature of 860.degree. C. is well above the deformation temperature of the large volume-type commercial glasses.